USING A CALCULATOR TO INVESTIGATE WHETHER A LINEAR, QUADRATIC OR EXPONENTIAL FUNCTION BEST FITS A SET OF BIVARIATE NUMERICAL DATA

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1 USING A CALCULATOR TO INVESTIGATE WHETHER A LINEAR, QUADRATIC OR EXPONENTIAL FUNCTION BEST FITS A SET OF BIVARIATE NUMERICAL DATA Jackie Scheiber RADMASTE, Wits University Jackie.scheiber@wits.ac.za Target audience: FET Band Mathematics teachers Duration: 2-hour workshop CURRICULUM REFERENCES 1) GRADE 9 (5.2 Represent data) Draw a variety of graphs by hand/technology to display and interpret data including: bar graphs and double bar graphs histograms with given and own intervals pie charts broken-line graphs scatter plots 2) GRADE 12 (10. Statistics) a) Represent bivariate numerical data as a scatter plot and suggest intuitively and by simple investigation whether a linear, quadratic or exponential function would best fit the data. b) Use a calculator to calculate the linear regression line which best fits a given set of bivariate numerical data. c) Use a calculator to calculate the correlation co-efficient of a set of bivariate numerical data and make relevant deductions. Reference: Pike, M. Classroom Mathematics Grade 12 (2013) Heinemann Publishers 1

2 BIVARIATE DATA When each item in a population has two measurements associated with it (e.g. the heights and the masses of the members of a rugby team), then the data can be represented by points on a two-dimensional scatter plot or scatter diagram or scatter graph or scatter chart. The scatter plot can show whether there is an association (or correlation) between the two variables. If the points are roughly in a line, then we can describe the correlation as being linear. A linear correlation can be described as positive if, as one quantity increases, so does the other OR negative if, as one quantity increases, the other decreases. y y x Positive linear correlation When the points in a scatter plot have a significant or very strong negative or strong positive correlation, we can draw a line of best fit, also called a least squares regression line or just a regression line. The line of best fit is a line about which the points appear to be clustered. Calculators and statistical software can be used to find the equation of the line of best fit or regression line. The bivariate data items are used as input. The calculator or computer then given the values of the unknowns in the equation. y x Negative linear correlation As can be seen alongside, in some cases a curve will fit a set of bivariate data better than a line can. We can use a calculator to calculate the equation of an appropriate non-linear function that models the data. At school level the two non-linear functions that we use are the quadratic and exponential functions. No correlation x One of the difficulties of using a curve to model the data is that the curve may fit the data within the given range, but may otherwise be inappropriate. It is always best to examine the scatter plot carefully and to then fit a curve that looks close to the data points and also seems to explain the relationship in the context of the data. Non-linear correlation 2

3 TASK 1 The table lists the world records for the 100 m men s sprint from 1983 to Athlete Nationality Year Time (s) 1. Willie Williams USA ,1 2. Armin Hary West Germany ,0 3. Jim Hines USA ,95 4. Calvin Smith USA ,93 5. Carl Lewis USA ,92 6. Leroy Burrell USA ,90 7. Carl Lewis USA ,86 8. Leroy Burrell USA ,85 9. Donovan Bailey Canada , Maurice Greene USA , Tim Montgomery USA , Asafa Powell Jamaica , Asafa Powell Jamaica , Usain Bolt Jamaica , Usain Bolt Jamaica , Usain Bolt Jamaica ,58 (Source: ) Asafa Powell powers to the 9,77 second World record in Athens in 2005 (AFP/Getty Images) 1) Use the data to draw a scatter plot on the given set of axes. 2) Describe the correlation between the two variables in words. Is there a direct cause and effect relationship between the variables or is it possible that the relationship between the two variables may be a coincidence? 3) a) Use your calculator to determine the linear (y = A + Bx) that best models the data. b) Use the calculator and the equation of the linear regression line to complete the following table: Time (s) c) Use the values in the table to draw the linear on the scatter plot. 3

4 4) a) Use your calculator to determine the quadratic (y = A + Bx + Cx 2 ) that best models the data. b) Use the calculator and the equation of the quadratic to complete the following table: Time (s) c) Use the values in the table to draw the quadratic on the scatter plot. 5) a) Use your calculator to determine the exponential (y= A.B x ) that best models the data. b) Use the calculator and the equation of the exponential to complete the following table: Time (s) c) Use the values in the table to draw the exponential on the scatter plot 6) Complete the following table by filling in the sets of values that you have calculated on it. Year Actual time (s) , , , , , ,69 Value of linear (s) Value of quadratic (s) Value of exponential (s) 7) Which suits the context best? Give reasons for your answer. 4

5 Time (in seconds) 1) SOLUTION TO TASK 1 Athlete Nationality Year Time (s) 1. Willie Williams USA ,1 2. Armin Hary West Germany ,0 3. Jim Hines USA ,95 4. Calvin Smith USA ,93 5. Carl Lewis USA ,92 6. Leroy Burrell USA ,90 7. Carl Lewis USA ,86 8. Leroy Burrell USA ,85 9. Donovan Bailey Canada , Maurice Greene USA , Tim Montgomery USA , Asafa Powell Jamaica , Asafa Powell Jamaica , Usain Bolt Jamaica , Usain Bolt Jamaica , Usain Bolt Jamaica ,58 WORLD RECORDS FOR THE 100 m MEN'S SPRINT FROM 1956 TO Year When The Record Was Set 2) Describe the correlation that exists between the two variables: 5

6 3) a) Calculate the linear (y = A + Bx) as follows: Go to the Stats Mode Enter all the X-values Go to the top of the Y-value column Enter all the Y-values Clear the screen Go to the Stats Calculation Screen and then find the values of A and B [MODE] [2:STAT] [2: A + BX] 1956 [=] 1960 [=] 1968 [=] 1983 [=] [ ] [ ] 10.1 [=] 10.0 [=] 9.95 [=] 9.93 [=] Press [AC] [SHIFT] [1] (STAT) [5: Reg] [1: A] [=] [SHIFT] [1] (STAT) [5: Reg] [2: B] [=] A =.. (correct to 2 decimal places B =.. (correct to 3 decimal places) Equation of the linear : b) Use your calculator to determine the time (correct to 2 decimal places) for the given years as follows: Determine the time in [SHIFT] [1] (STAT) [5: Reg] [5: ] Determine the time in [SHIFT] [1] (STAT) [5: Reg] [5: ], Continue like this to determine the rest of the values Time (to 2 decimal places) c) Plot these values onto the scatter plot and use them to draw the linear regression function on your scatter plot. 6

7 4) a) Calculate the quadratic (y = A + Bx + Cx 2 ) as follows: NOTE: you don t have to enter the data again. Change to the quadratic [SHIFT] [1] (STAT) [1: Type] [3: _ + CX 2 ] Clear the screen Press [AC] Find the values of A, B and C [SHIFT] [1] (STAT) [5: Reg] [1: A] [=] [SHIFT] [1] (STAT) [5: Reg] [2: B] [=] [SHIFT] [1] (STAT) [5: Reg] [3: C] [=] A =.. (correct to 2 decimal places B =... (correct to 2 decimal places) C =... (correct to 4 decimal places) Equation of the quadratic : b) Use your calculator to determine the time (correct to 2 decimal places) for the given years as follows: Determine the time in [SHIFT] [1] (STAT) [5: Reg] [6: ] Determine the time in [SHIFT] [1] (STAT) [5: Reg] [6: ], Continue like this to determine the rest of the values Time (to 2 decimal places) c) Plot these values onto the scatter plot and use them to draw the graph of the quadratic. 7

8 5) a) Calculate the exponential (y = ) as follows: NOTE: you don t have to enter the data again. Change to the exponential [SHIFT] [1] (STAT) [1: Type] [6: A. B^X] Clear the screen Press [AC] Find the values of A and B [SHIFT] [1] (STAT) [5: Reg] [1: A] [=] [SHIFT] [1] (STAT) [5: Reg] [2: B] [=] A =.. (correct to 2 decimal places B =... (correct to 3 decimal places) Equation of the exponential : b) Use your calculator to determine the time (correct to 2 decimal places) for the given years as follows: Determine the time in [SHIFT] [1] (STAT) [5: Reg] [5: ] Determine the time in [SHIFT] [1] (STAT) [5: Reg] [5: ], Continue like this to determine the rest of the values Time (to 2 decimal places) c) Plot these values onto the scatter plot and use them to draw the graph of the exponential. 6) Fill in your calculated values on this table: Year Actual time (s) , , , , , ,69 Value of linear Value of quadratic Value of exponential 7) Which suits the context best? Give reasons for your answer. 8

9 TASK 2 Felix Baumgartner of Austria holds the record for high altitude skydiving. In October 2012 he skydived an estimated 39 km. His highest speed reached before the air resistance started slowing him down, was km/h or 373 m/s which is faster than the speed of sound. The following table gives the speed, v in m/s, at which a skydiver is moving after having travelled a through a distance, s in m, from a helium balloon which was at a great height when the skydiver jumped. elix-baumgartner-skydive-youtube Distance s (m) Speed v (m/s) ) Draw a scatter plot of the data 2) Use your calculator to fit the following s to the data: a) Linear b) Exponential 3) Sketch the graphs of the functions obtained in 2) on to your scatter plot using different colours for each. 4) Use the two models and the calculator to determine the speed of the skydiver after falling through a distance of a) m b) m c) m 5) Which of the two models fits the data better? Give reasons for your choice. 9

10 Speed in m/s SPEED AND HEIGHT OF A SKYDIVER JUMPING FROM A HELIUM BALLOON Distance in metres 10